Backup reminders: include all Get Paid derived wallets in device-key total balance

[BullishNode]

Upstream PR SatoshiPortal#2453 introduces backup reminders whose 10M-sat first-crossing trigger reads the total balance of wallets whose keys are stored on the device (locally-signing mainnet + Ark; watch-only/watch-signer/hardware excluded).

Upstream has no Get Paid wallets. In this fork, the BIP85-derived product wallets (Lightning Address, Payment Page, POS, BTCPay — derived from the default seed, keys on device) MUST be included in that total when the Get Paid stack merges over the redesigned backup feature.

Acceptance:

• Audit the reminder engine's balance source and confirm every derived wallet with device-stored keys contributes to the total.
• Test: crossing 10M sats via a Get Paid derived wallet balance alone fires the one-time crossing reminder.
• Document the fork's balance definition next to the upstream one (docs/backup-health-reminders.md).

Context: owner decision 2026-07-27 — reminder engine is: vault-only → 90d; physical-done → 365d since last test; one-time first-crossing of 10M sats regardless of posture; dismiss = snooze to next cycle; no balance tiers, no escalation. Design doc: ~/BACKUP_SCREEN_REDESIGN.md.